This invention relates generally to a system and method for measuring the doping levels and doping profiles of a semiconductor substrate, and in particular to a system and method for measuring the doping level or doping profile of a semiconductor substrate in a semiconductor fabrication line environment.
In the fabrication of electrical active or passive devices on a semiconductor substrate, one of the most critical processes is a doping process in which impurity atoms, such as boron, phosphorus, or arsenic, are imbedded into the semiconductor substrate using various methods, such as ion implantation or diffusion. These impurities are imbedded within the semiconductor substrate, and the area is known as a doped region, and the doped region has a certain doping profile. The doping profile is the doping levels of the doped region at various points in the area of the doped region. The impurities within the semiconductor substrate determine the semiconductor substrate's conductivity, which is how well charge carriers move through a substrate. The careful control of this doping profile is critical in order to obtain an active or passive device, such as transistors, diodes, resistors, or capacitors, on the semiconductor substrate with desired electrical properties.
As indicated above, there are two conventional methods for forming a doped region in a semiconductor substrate. These methods form doped regions with profiles that may differ. In the ion implantation method, impurity atoms are accelerated, strike the surface of the semiconductor substrate, and penetrate the surface where the impurity atoms collide with the substrate lattice and stop. The substrate is then annealed (i.e, heated to high temperatures on the order of 1000.degree. C.). The doping profile for an ion implantation method is gaussian in shape (i.e., a curve with a peak near the surface of the substrate). In the diffusion method, the substrate is heated to a high temperature in a chamber that also contains a gas that has impurity atoms suspended in the gas. The impurity atoms in the gas diffuse into the substrate, and form a doping profile that is described by a complementary error function, which is well known function. The doping region may also be generated by a combination of methods where, for example, the substrate surface may be heavily doped with impurity atoms at low energy levels, and then the impurity atoms are diffused into the substrate by heating the substrate.
It is desirable to be able to accurately measure the doping profile of a device, whether it is active or passive, formed on the semiconductor substrate. It is also desirable to measure doping profiles of a semiconductor substrate during fabrication of the substrate to permit real-time control of a fabrication line and to vary the fabrication process. There are a number of conventional methods for determining the doping profile. One is a spreading resistance method in which probes are placed on the surface of the substrate, and the voltage drop across the probes is measured in response to an applied current. This method for measuring the doping profile requires too much area of the substrate surface so that is cannot be used with production substrates and cannot be used for real time control of a fabrication line. In addition, the method requires that the probes directly contact the substrate surface, which is undesirable.
Another method of measuring the doping profile of the semiconductor substrate is known as capacitance-voltage (C-V) profiling in which a junction is formed at the substrate surface, biased with a voltage, and a measurement of the capacitance of the junction as a function of the voltage applied across the junction permits the doping profile to be measured. This method also requires that probes are connected to the substrate and has the same drawbacks as the spreading resistance probe method.
The doping profile may also be measured indirectly by a method, known as Thermaprobe, in which the substrate is heated by a laser beam, and the response of the substrate to the heating is measured. This method is normally used after ion implantation, but only measures the damage caused to the substrate by the ion implantation, not the doping profile of the substrate. This method also is used prior to annealing to make the impurity atoms electrically active. Some other conventional methods include Hall effect measurements, Fourier transform infrared determination, secondary ion mass spectrometry, and plasma resonance.
None of these conventional methods of measuring the doping profile of the semiconductor substrate can be used as process monitors for real-time control in a fabrication line to measure the doping profiles of semiconductor substrate devices as they are being manufactured. The conventional methods are difficult to use and many require as area of the substrate that is so large as to preclude use of the method on production substrates in a fabrication process. They also have a limited range of resolutions that cannot measure the doping levels of the smaller semiconductor devices that are being manufactured, and may require direct contact with the surface of the substrate. Thus, these systems cannot be used as process monitors for real-time control of a fabrication line.
Therefore, there is a need for a system and method for real-time monitoring and measurement of doping profiles of doped regions in a semiconductor substrate to control the fabrication process which avoid these and other problems of known systems and methods, and it is to this end that the invention is directed.